Stroke is a major cause of death and disability and brain blood vessels have been implicated. Stroke outcome is worsened by apoptotic death of endothelial cells (EC), failure of angiogenesis and insufficient growth of collateral vessels. In brain EC, sphingosine-1 -phosphate (S1P) activates Akt and telomerase, suppresses apoptosis and induces proliferation. Neurons, and possibly glial cells, are a source of S1P in ischemia because the S1P synthesizing enzyme (sphingosine kinase 2, SPK2) is upregulated in neurons following oxygen glucose deprivation and middle cerebral artery occlusion (MCAo) in mice. We hypothesize that following ischemia, neuron/glia-derived S1P exerts anti-apoptotic and pro-angiogenic effects on EC via telomerase. Two (2) specific aims will test the hypothesis that neuron/glia-derived S1P activates EC S1P1 receptors, Akt and telomerase, protecting EC from apoptosis and inducing neovascularization. Aim 1 (in vitro) will confirm and characterize the telomerase responses (activation and up-regulation) to S1P in EC, and test the hypothesis that S1P produced by neuronal and glial SPK2 induces telomerase activation, thereby mediating EC protection and proliferation. We propose that S1P1 receptors, Akt and Heat-Shock Protein 90 (HSP90) play a key role in these effects. Aim 2 (in vivo) will show that the S1P receptor agonist prodrug FTY720 increases telomerase expression in brain EC and extend preliminary findings showing that this agent decreases infarct size when administered before or after MCAo. Using transgenic mice expressing dominant negative Akt under the control of an EC-specific promoter, or mice lacking either the catalytic subunit or the RNA template of telomerase, we will test the hypothesis that the protective effect of FTY720 is related to Akt and telomerase activation in EC, and that this effect does not depend on the catalytic activity of telomerase (FTY720 should protect in RNA template knockouts).This project will study a novel system that can provide long-lasting improvement in EC function following stroke and serve as a new target for stroke therapy.